Update supported extractors

documentation.html

@@ -72,9 +72,22 @@ Unsupported, see https://github.com/rg3/youtube-dl/issues/300
 <li>The Escapist.</li>
 <li>CollegeHumor.</li>
 <li>arte.tv.</li>
+<li>Soundcloud.</li>
+<li>xvideos.</li>
+<li>infoq.</li>
+<li>mixcloud.</li>
+<li>Stanford Open Content.</li>
+<li>Youku.</li>
+<li>MTV.</li>
+<li>XNXX.</li>
+<li>Google Plus.</li>
 <li>A generic downloader that works in some sites.</li>
 </ul>
 
+<p>
+For a complete list, execute <code>youtube-dl --list-extractors</code>.
+</p>
+
 <h1 id="d5">Proxy support</h1>
 
 <p>youtube-dl supports downloading videos through a proxy, by setting the <em>http_proxy</em> environment variable to the proxy URL, as in <code>http://proxy_machine_name:port/</code>.</p>